refactor: move ErrorWrapperResolver to errorsx pkg (#419)

Part of https://github.com/ooni/probe/issues/1505

internal/errorsx/resolver.go

@@ -0,0 +1,52 @@
+package errorsx
+
+import "context"
+
+// Resolver is a DNS resolver. The *net.Resolver used by Go implements
+// this interface, but other implementations are possible.
+type Resolver interface {
+	// LookupHost resolves a hostname to a list of IP addresses.
+	LookupHost(ctx context.Context, hostname string) (addrs []string, err error)
+}
+
+// ErrorWrapperResolver is a Resolver that knows about wrapping errors.
+type ErrorWrapperResolver struct {
+	Resolver
+}
+
+var _ Resolver = &ErrorWrapperResolver{}
+
+// LookupHost implements Resolver.LookupHost
+func (r *ErrorWrapperResolver) LookupHost(ctx context.Context, hostname string) ([]string, error) {
+	addrs, err := r.Resolver.LookupHost(ctx, hostname)
+	err = SafeErrWrapperBuilder{
+		Classifier: ClassifyResolveFailure,
+		Error:      err,
+		Operation:  ResolveOperation,
+	}.MaybeBuild()
+	return addrs, err
+}
+
+type resolverNetworker interface {
+	Network() string
+}
+
+// Network implements Resolver.Network.
+func (r *ErrorWrapperResolver) Network() string {
+	if rn, ok := r.Resolver.(resolverNetworker); ok {
+		return rn.Network()
+	}
+	return "errorWrapper"
+}
+
+type resolverAddresser interface {
+	Address() string
+}
+
+// Address implements Resolver.Address.
+func (r *ErrorWrapperResolver) Address() string {
+	if ra, ok := r.Resolver.(resolverAddresser); ok {
+		return ra.Address()
+	}
+	return ""
+}